I do hope that somebody can answer your query with some authority. I believe that it is illegal to keep your engine running when parked or at a standstill other than in traffic but so many drivers do it that I question my understanding of the situation.
Diesels are the worst with their known carcinogenic pollutants and it seems a lot of the drivers think it is their right to keep the engine running in the winter to keep warm and in the summer to keep cool, as a group coach drivers win the gold.

'Idling engines
7.14 Vehicle driver who leave their engine running when unnecessary make a contribution to air
pollution. It is an offence under Regulation 98 of the Road Vehicles (Construction and Use)
Regulations 1986 for vehicles left idling unnecessarily whilst stationary. The Road Traffic
(Vehicle Emissions) Regulations 1986 for vehicles left idling unnecessarily whilst stationary.
The Road Traffic (Vehicle Emissions) (Fixed Penalty)(England) Regulations 2002 enable
local authority authorised persons to request vehicle users to switch off engines when parked
and to issue fixed penalty notices to those who refuse to co-operate. It is a requirement of
Regulation 88 of the Road vehicles (Construction and Use) Regulations 1986, as amended,
that drivers switch off engines in parked vehicles. Fixed penalty fines of £20 can be issued in
such instances
7.15 There are several locations within Southwark where drivers tend to leave their engines
running, these are primarily bus and coach stands but also outside schools. We will support
the Mayor’s policy to make London a ‘no idling zone’ and undertake a publicity campaign
followed by enforcement to reduce this activity in Southwark.
Measure 4:
Southwark will, following a publicity campaign, undertake enforcement on idling engines at
hotspots within the borough.'

No idea if it's been put into practice at any particular 'hotspot' or if there has ever been the preliminary publicity campaign. Over to you, councillors?

In bartholomew street an average of thirty plus coaches park up very day, it is a continuous stream of coaches, red,blue,white,yellow all colours of the spectrum. They park on yellow lines with impunity, even on the brow of the red lines new kent road end. The council have plant ed two young trees, better if they were signs reminding drivers to turn off engines. I don't want to stop drivers having a break, but a certain coach company leave their red coaches there all day. If the engines are running I go and tell them to turn them off, if the drivers have attitude I phone the company.

'Idling engines
7.14 Vehicle driver who leave their engine running when unnecessary make a contribution to air
pollution. It is an offence under Regulation 98 of the Road Vehicles (Construction and Use)
Regulations 1986 for vehicles left idling unnecessarily whilst stationary. The Road Traffic
(Vehicle Emissions) Regulations 1986 for vehicles left idling unnecessarily whilst stationary.
The Road Traffic (Vehicle Emissions) (Fixed Penalty)(England) Regulations 2002 enable
local authority authorised persons to request vehicle users to switch off engines when parked
and to issue fixed penalty notices to those who refuse to co-operate. It is a requirement of
Regulation 88 of the Road vehicles (Construction and Use) Regulations 1986, as amended,
that drivers switch off engines in parked vehicles. Fixed penalty fines of £20 can be issued in
such instances
7.15 There are several locations within Southwark where drivers tend to leave their engines
running, these are primarily bus and coach stands but also outside schools. We will support
the Mayor’s policy to make London a ‘no idling zone’ and undertake a publicity campaign
followed by enforcement to reduce this activity in Southwark.
Measure 4:
Southwark will, following a publicity campaign, undertake enforcement on idling engines at
hotspots within the borough.'

No idea if it's been put into practice at any particular 'hotspot' or if there has ever been the preliminary publicity campaign. Over to you, councillors?

...so, to answer Zoe's question 'Do councils have powers to fine drivers who leave engines running? I think they should, but I suspect they don't.' the answer seems to be:

Only if the council have identified a 'hotspot' AND carried out a 'publicity campaign' AND (I assume, although it isn't mentioned) erected warning signs alerting motorists to the fact that they face an on-the-spot fine if they don't turn off their engines.

And that also seems to answer Julia T's original query - it's nothing to do with the police, and traffic wardens can only take action if the council has gone through all the legal hoops first.

Yes - if they had to go around stopping thoughtless drivers from polluting the environment, they'd have no time for their more important work, like spying on the grieving parents of murder victims, (allegedly) taking bungs from national newspapers in return for invading the privacy of private individuals and covering up the killing of newspapermen, Brazilian electricians and teachers.

And that also seems to answer Julia T's original query - it's nothing to do with the police, and traffic wardens can only take action if the council has gone through all the legal hoops first.[/quote]

Thank you to everybody who explained the legal situation and contributed to the discussion. Very helpful and informative, many thanks!

Well, I reckon it is now up to us the voters and local council tax payers to let the decision makers know how much we care, or not, about the air quality we and our children breath in. If this becomes a pressing concern for law makers hopefully can be supported by active citizens.